Conventional facsimile transmission systems sequentially scan pictures (images) line-by-line, from top to bottom and transmit the information to a receiver having a hard-copy output device which sequentially reproduces the picture in the same manner. When facsimile transmitters and receivers are connected over low capacity transmission channels the transmission of high resolution pictures can take a long time. User interaction in such a transmission environment can be frustrating, especially when the user has to browse through a series of pictures.
The utilization of cathode-ray tube monitors and other soft copy output devices together with frame memories now enable additional capabilities to be added to these facsimile transmission systems. One capability is that modification and updating of the displayed picture is possible and, therefore, it is not necessary to transmit the picture in the usual sequential line-by-line format. Progressive facsimile transmission systems utilize this capability to permit the transmission and display of a coarse version of a picture in a small fraction of the total transmission time. Subsequent transmissions gradually improve the picture quality to that of the original picture. In progressive transmission it is important that the most important preceptual features of the picture be transmitted first and in a compressed form. Additionally, the time required for the transmission of all the progressive transmissions should not be much higher than the time required for a conventional line-by-line transmission.
A paper entitled "Progressive Refinement of Raster Images" (published in the IEEE Transaction on Computers, Vol. C-28, No. 11, Nov. 1979) by K. R. Sloan and S. L. Tanimoto reviews several progressive image transmission techniques which use a pyramidal data structure. Those pyramidal data transmission techniques send successive levels of signals representing successively smaller arrays of picture elements. In U.S. Pat. No. 4,222,076, issued on Sept. 9, 1980 to K. C. Knowlton and entitled "Progressive Image Transmission", another pyramidal data structure technique is disclosed. A paper entitled "Progressive Image Transmission Using a Growth-Geometry Coding" (published in the Proceedings of the IEEE, Vol. 68, No. 7, July 1980) by A. J. Frank, J. D. Daniels and D. R. Unangst describes a growth-geometry coding technique for progressive image transmission.
While equipments utilizing the above techniques are efficient, they are incompatible with the line-by-line codes which have been proposed as the standard for conventional digital facsimile transmission over the general switched telephone network. Currently, line-by-line codes such as the one dimensional (1D) run length code called the "Modified Huffman Code" (MHC) and the two-dimensional (2D) extention of the MHC called the "Modified READ Code" (MRC) are proposed as candidates for the standard code. The paper entitled "International Digital Facsimile Coding Standards" (published in the Proceedings of the IEEE, Vol. 68, No. 7, July 1980) by R. Hunter and A. H. Robinson describes the MHC (1D) and MRC (2D) codes which description is incorporated herein by reference. However, as noted previously, the use of these line-by-line codes in a top to bottom sequentially scanned facsimile system is undesirably slow for use with low capacity transmission channels.
The art is lacking an efficient progressive transmission technique which utilizes the above line-by-line codes to encode the picture information for transmission over low capacity channels.